fn main() {
    // let s1 = String::from("hello");
    // &符号是引用,*符号是解引用
    // let len = calculate_length(&s1);
    // println!("The length of '{}' is {}.", s1, len);

    let mut s = String::from("hello");
    change(&mut s);
}

// fn calculate_length(s: &String) -> usize {
//     s.len()
// }

fn change(some_string: &mut String) {
    some_string.push_str(", world");
}
